As long don't go above gtx 1070 can already. Max is a gtx 1070. And not everyone is comfortable buying 2nd hand or used gpus. Buying used gpu carry a certain risk as some are mining cards and you don't know how the previous owner have abuse the card. I.e whether he overclock or not etc. Seller can tell you he light usage only but yo believe or not is also another thing. He can tell you light usage just so he can sell you the card but in actual fact he use it for mining or game 24/7 you also don't know. Worst if the card left only with less then 1 year warranty can be quite risky to buy as the card can die any moment after the warranty period.
Buying a brand new card ensures that you are getting the full warranty period (3 or 5 years warranty) and also ensures that the card is not previously abuse or heavily use or worse (use for mining) by the previous users hence reducing the life span of the card.
I have gpus that die at exactly 3 years period just nice exactly after the 3 years warranty. Gtx 780, R9 270 are some that die after 3 years.
While its nice to be able to buy second hand 1070 ti at $300 -$400. I would prefer to buy brand new gtx 1060 or gtx 1660 or rx 580, rx 570 for $200-$3xx. That's just me.
If you are willing to take the risk of buying second hand gpus go ahead. For short period of usage is fine but if going to use for very long might not be good idea.
But then it really depends my past experience is that I could find some 2nd hand cards that is only 2-3 months old and selling at really cheap prices. Those can buy and is a good deal because still got official local warranty of 2 to 4 years warranty to cover.
But for 2nd hand cards that is very old say 2-3 or 4 years old cards with only 1 year or few months warranty left those I will stay away from. And also those mining cards.